I'm really starting to feel like an idiot here.
Not really sure what you're trying to beat me over the head with.
It looks like four different battery options...?
Is that right?
Is the C-number relevent here or are they all the same?
Knowing our requirements, is there one combination you would recommend over another?
If it's all the same then I would think the fewer number of pieces the fewer connections we'd have to make.
And what about charging?
The IMax B6?
Does that go between the panels and the battery?
What A/C charger were you thinking of or should we just use the panels leading up to the race?
I'm starting to get discouraged here, it seems like I'm missing something... something obvious.
<sigh>

Sona 18650 cells may not qualify due to lack of documentation-verification of capacity.

2 different Lipo possibilities.
Higher C rate provides less sag but very high C rate tends to sacrifice reliability-durability.
Under your planned discharge C rate will be less than 3 C, so possibly 20-30C might be optimal.

I know it takes a lot of energy to get the prop started
So what is the amp draw of the controller and add a few ? So 12s 2p of Samsung 25r cells 25amp cell. Price is going down. Maybe 3.75- 4.00 usd each.

How will we ensure 25 minutes out of this set-up?
If we fall short by even 20 seconds we are disqualified!

"I know it takes a lot of energy to get the prop started"Not really, especially considering the high torque of electric motors on start up.
It takes considerable more power at speed, ~8 - 10 knots ( ~4.5 m/s ).

Yes, we are advised to have a second battery in case of poor weather and the solar-only heats will run on battery instead.

But I thought you didn't like the first one, the HobbyKing R/C one.
Graphene you say. Perhaps that has something to do with the differing three letter abbreviation associated with the different lipo listings.

Come on, stick your neck out and recommend one... please.
Or your two top picks, then you're not on the hook.
Is the heavier battery, of the same rating, the better way to go?

With them RC lipo its just like lego, you plug whatever you want together, making sure to pay very very very very close attention to what you are doing. Buying or making a parallel adapter, I used a 5.5mm connector with 10awg that split into six 4mm 12 or 14awg wire, then I bought some 5.5mm bullets and some wire to solder the main wire. Its pretty simple to learn. Then you got to take care in charging and discharging. One day I will buy LiPo again.

999zip999 wrote:I know it takes a lot of energy to get the prop started
So what is the amp draw of the controller and add a few ? So 12s 2p of Samsung 25r cells 25amp cell. Price is going down. Maybe 3.75- 4.00 usd each.

Did you mean it takes a lot to get the motor going?
I've been told we may have a problem when running on panels only getting the motor turning due to it's high start-up draw. The panels may drop below the min voltage cut off in the controller.

Can we pick the direction of rotation on this motor or is it predetermined?
We are proposing to thread an aluminum tube onto the shaft threads of the motor.
If it turns counterclockwise, it will tend to unscrew our propeller shaft!